A flat type color cathode ray tube is provided with an electron gun which is extended along the direction parallel to the surface of a phosphor screen to make an envelope flat. The flat type cathode ray tube of this kind includes a flat tube envelope 1 as shown in FIGS. 1 and 2. This tube envelope 1 comprises, for example, a glass panel portion 1a, a glass funnel portion 1b, which forms a flat cavity 2 between the former and the latter and is made narrower as it comes closer to one side, namely, made as the form of a funnel (funnel shaped), and a glass neck portion 1c which is located at one narrow side thereof to communicate with the flat cavity 2.
Within the flat envelope 1 are placed a phosphor screen 3 and an opposing electrode 4 facing to the phosphor screen on its flat surface in the flat cavity 2. Both of them are placed in parallel to each other relative to the direction perpendicular to the flat surface of the tube envelope 1. On the inner surface of the panel portion 1a, for example, of the tube envelope 1 are deposited a target electrode 5 made of, for example, a transparent electrode and the phosphor screen 3 and the opposing electrode 4 made of, for example, a metal plate is located on the inner surface of the funnel portion 1b to oppose the former.
The phosphor screen 3 comprises stripe or dot like predetermined phosphor patterns which will emit, for example, red, green and blue light. In facing relation to this phosphor screen 3, an electrode 13 which determines an electron beam landing position, for example, aperture grille or shadow mask and the like is located to allow electron beams corresponding to respective color, which will be described later, to land on the phosphors of corresponding colors.
On the other hand, an electron gun 7 is located within the neck portion 1c, which is arranged such that electron beams emitted from the electron gun pass through the substantially center between the phosphor screen 3 and the opposing electrode 4 and then extends along the direction parallel to the surface of the phosphor screen 3.
The electron gun 7 can be constructed as a multi-beam single electron gun in which, as shown in FIGS. 3 and 4, three cathodes K.sub.R, K.sub.G and K.sub.B corresponding to, for example, red, green and blue colors are arranged on the horizontal plane, namely, in line with one other. A first grid G.sub.1, a second grid G.sub.2, a third grid G.sub.3, a fourth grid G.sub.4 and a fifth grid G.sub.5 which are common thereto are located in turn. The third to fifth grids G.sub.3 to G.sub.5 constitute a main electron lens L of, for example, the unipotential type and a convergence means C is located at the rear stage of the fifth grid G.sub.5. The convergence means C comprises a pair of inner deflection plates C.sub.1 and C.sub.2 which are arranged symmetrically on both sides of the axis of the electron gun 7, namely, on the plane substantially perpendicular to the phosphor screen and are symmetrical to each other in the longitudinal direction relative to the horizontal plane passing through the axis of the electron gun 7. Outside the respective deflection paltes C.sub.1 and C.sub.2 located are a pair of outer deflection plates C.sub.3 and C.sub.4, each of which is opposed in parallel relation to the deflection plates C.sub.1 and C.sub.2, are similarly arranged along the above mentioned plane perpendicular to the phosphor screen and are symmetrical to each other on both sides of the axis of the electron gun. In addition, they are arranged symmetrical to each other in the longitudinal direction relative to the horizontal plane passing through the axis of the electron gun. The pair of inner deflection plates C.sub.1 and C.sub.2 are electrically coupled to the fifth grid G.sub.5 of the last stage to which a high voltage is applied. Between the inner deflection plates C.sub.1, C.sub.2 and the outer deflection plates C.sub.3 and and C.sub.4 applied is a deflection voltage.
A high anode voltage is applied to a target electrode 5, namely, the phosphor screen 3 and a high voltage lower than the above anode voltage are applied to the opposing electrode 4, thus forming a first deflection field between the phosphor screen 3 (the target electrode 5) and the opposing electrode 4.
A second deflection field is constructed between the electron gun 7 and the position of the phosphor screen 3. The second deflection field deflects the electron beams emitted from the electron gun 7, for example, three electron beams b.sub.R, b.sub.G and b.sub.B in the horizontal and vertical directions. The horizontal deflection is such deflection that the electron beam from the electron gun 7 is deflected in the direction substnatially perpendicular to the axial direction of the electron gun 7 and in the direction parallel to the surface of the phosphor screen 3 to perform a so-called horizontal scanning on the phosphor screen 3. Meanwhile, the vertical deflection is such deflection that the same beam is deflected in the direction perpendicular to the horizontal deflection to perform a vertical scanning on the phosphor screen 3. Reference numeral 8 designates a deflection means which forms the second deflection field. The horizontal deflection which requires, for example, a relatively large deflection angle is carried out by the electromagnet deflection, while the vertical deflection is carried out by the electrostatic deflection. This deflection means 8 is electromagnet and electrostatic deflection type.
The deflection means 8, as shown in FIGS. 1 and 2, consists of an annular magnetic core 9 made of, for example, ferrite having high magnetic permeability surrounding the outer periphery of the tube envelope 1 at the rear stage of the electron gun 7, an electromagnet coil 10 passing therethrough the horizontal deflection current and a pair of deflection plates 11a and 11b made of, for example, high magnetic permeability magnetic material such as Mn--Zn ferrite, Ni--Zn ferrite or the like whitin the tube envelope 1 to serve as the inner pole pieces and electrostatic deflection plates.
The deflection plates 11a and 11b are located to oppose to each other in the direction perpendicular to the flat surface of the tube envelope 1 at the both sides of the passage of the electron beam, namely, located in parallel to the opposing electrode 4 and the phosphor screen 3. The magnetic core 9 is formed as the annular shape surrounding the outer periphery of the tube envelope 1 and includes outer center poles 12a and 12b which grip the deflection plates 11a and 11b within the tube envelope 1 to project to the inside so as to oppose to each other. Around the outer peripheries of the outer center poles 12a and 12b is wound at least one of coils 10a and 10b. With the construction thus made, the horizontal deflection current is flowed to the coil 10 (10a and 10b) thereby to establish between both the outer center poles 12a and 12b and further between the inner pole pieces and electrostatic deflection plates 11a and 11 b existing therebetween the horizontal deflection magnetic field which transverse the passage of the electron beam in the direction perpendicular to the flat surface of the envelope 1. On the other hand, the vertical deflection signal voltage is applied between the deflection plates 11a and 11b to thereby establish the electrostatic vertical deflection field to the passage of the electron beam in the direction perpendicular to the flat surface of the envelope 1.
The electron beams b.sub.R, b.sub.G and b.sub.B emitted from the respective cathodes K.sub.R, K.sub.G and K.sub.B of the electron gun 7 intersect with one another at substantially the center of the main electron lens L and then pass therethrough. After that, the electron beams b.sub.R, b.sub.G and b.sub.B are diverged and travelled through between the deflection plates C.sub.2 and C.sub.4, C.sub.1 and C.sub.2, C.sub.1 and C.sub.3 of the convergence means C. The deflection voltage applied between the inner deflection plates C.sub.1, C.sub.2 and the outer deflection plates C.sub.3, C.sub.4 permit three beams b.sub.R, b.sub.G and b.sub.B to be concentrated (converged) on substantially the phosphor screen 3. Strictly speaking, three beams b.sub.R, b.sub.G and b.sub.B are converged at a beam throughhole of the electrode 13 which determines the electron beam landing position which is located to face the phosphor screen 3. Due to the differences of the incident angles of the beams b.sub.R, b.sub.G and b.sub.B on this electrode 13, the beams b.sub.R, b.sub.G and b.sub.B are respectively landed on the phosphors of the corresponding colors of the phosphor screen 3. On the other hand, since these electron beams b.sub.R, b.sub.G and b.sub.B emitted from the electron gun 7 are passed through the second deflection system generated by the horizontal and vertical deflection means 8, they are deflected in the horizontal and vertical directions. Further, these electron beams are deflected in the direction towards the phosphor screen 3 by the first deflection system established between the target electrode 5 (the phosphor screen 3) and the opposing electrode 4 at the rear stage. The cooperation of the first and second deflection systems allows the electron beams b.sub.R, b.sub.G and b.sub.B to scan the phosphor screen 3 in the horizontal and vertical directions. As described above, the color image produced on the phosphor screen 3 by the scanning of the electron beams is observed from the side of, for example, the panel 1a.
When the main electron lens is made common, each beam is arranged on the same plane and the concentration of each beam near the phosphor screen is performed on the surface perpendicular to the axis of the electron gun, the construction of the electron gun becomes simple. However, as described above, when this electron gun is applied to the flat type cathode ray tube in which the electron gun is located in the direction parallel to the phosphor screen, the travelling distance of the electron beam becomes considerably different relative to the vertical scanning direction of the phosphor screen. Namely, when each beam is converged at the beam throughhole of the electrode 13 which determines the beam landing position in a certain place in the vertical scanning direction of the phosphor screen, the beam is not converged at the beam through-holes in other places. For example, when each beam is exactly converged at the center of the phosphor screen 3, in the portion of the phosphor screen 3 farthest from the electron gun 7, each beam is converged in fron of the electrode 13, while in the portion of the phosphor screen nearest to the electron gun 7, each beam is converged behind the electrode 13. As a result, each beam is mislanded. Therefore, a socalled dynamic convergence compensation is necessary for changing the converging position of each beam in accordance with the change of the scanning position.